iStock/Thinkstock(NEW YORK) — Wall Street closed mixed as investors began to question the uncertainty around President-Elect Donald Trump’s policies after a post-election rally.

The Dow gained 21.03 (+0.11 percent) to finish at 18,868.69.

The Nasdaq fell 18.72 (-0.36 percent) to close at 5,218.40, while the S&P 500 finished at 2,164.20, down 0.25 (-0.01 percent) from its open.

Crude oil gained less than 1 percent with prices over $44 a barrel.Winners and Losers: News of a $4.5 billion merger between Siemens AG and Mentor Graphics Corp. pushed Mentor shares to soar 18 percent.

Shares in Harman International Industries Inc. climbed 25 percent after Samsung Electronics said it would acquire the U.S. auto-parts supplier for $8 billion.

Dynavax Technologies Corp’s hepatitis B vaccine was rejected by the U.S. Food and Drug Administration for the second time in three years, causing shares to plunge nearly 65 percent.

Looking Ahead: Federal Reserve Chair Janet Yellen is set to testify before Congress this week, updating the country on the status of the economy and whether or not there could be an interest rate hike in December. Given the market rally after the election, some experts believe the rate raise is likely to come next month.